1. Field of the Invention
This invention relates to a method and apparatus for forming a picture image by successively superposing a plurality of toner images for use in such processes as electrophotographing, computer output, facsimile, transmission recording and laser printers.
2. Description of the Prior Art
Multiple reference toner images in the following description are meant to designate those formed independently at different positions as different color reference toner images and at the same time as those formed with layers thereof at a position common to them.
In a method and apparatus for forming a plurality of toner images, for instance, a method and apparatus for forming a multicolor image, the image formed and recorded should be such that its image portion offers a high color density, a gradient faithful to the document being copied, and an excellent color balance, whereas the non-exposed portion is free from photographic fog. Japanese Patent Laid-Open No. 49-127629 and No. 50-20730 disclose methods for forming a reference toner image on an image retainer using a reference patch and controlling multicolor image quality according to the data derived from the reference toner image. In those known methods, a multicolor image is formed in the following order: For instance, a document and a reference patch are mounted on a document glass plate in such a manner that the latter is so positioned as to receive beams from a beam source prior to the former and the beams reflected from the reference patch and the document for image exposure are applied through a blue filter on an image retainer uniformly charged to form reference and picture latent images. Subsequently, a developing device containing yellow toner is used to develop and form a yellow reference toner image; and a yellow toner picture image is then formed based on the data derived from the reference toner image. The yellow toner picture image is transferred to recording paper supplied from a paper feeder at proper timing. The reference toner image and the residual toner image are removed from the image retainer and, after the residual charge has been eliminated, the image retainer is uniformly charged again to form reference and picture latent images by means of image exposure through a green filter. A developing device containing magenta toner is used in the same manner as above to develop a reference latent image and form a magenta reference toner image and a magneta toner picture image based on the data derived from the reference toner image. This toner image is piled on the yellow toner picture image already transferred to the recording paper in the preceding process. A cyan toner picture image is also transferred likewise and three primary color toner images are thus superposed. The recording paper is separated from a transfer drum and fixed before being discharged out of the apparatus.
Although the above method of forming a picture image has an advantage in that a multicolor image with improved image characteristics can be obtained, it also poses problems in that (i) the apparatus is large, and the time required for forming an image is lengthened because a transfer drum is used to transfer toner images to the recording paper each time the development of each color is completed; and (ii) misalignment in printing each color toner image because the repetition of the development and transfer process many times causes the color image formed to be out of register.
Accordingly, there has been proposed by, for instance, Japanese Patent Laid-Open No. 56-144452, Japanese patent application Nos. 58-184381 and 58-183152 a technique of superposing a plurality of toner picture images on an image retainer by means of reversal development and simultaneously transferring the image formed to recording paper.
Referring to a flowchart of FIG. 1, the typical image-forming principle of such a technique will be described subsequently.
In FIG. 1, there is shown a drum-shaped photo-sensitive member S together with positive surface potential E provided on the surface of the photo-sensitive member S, an image exposure portion PH of the photosensitive member S, a non-exposure portion DA of the photosensitive member S, an increase DUP in the potential because of the positively charged toner T adhering to the exposure portion PH in the first development and an increase CUP in the potential of the exposure portion PH because of the second charging.
The photosensitive member S is uniformly charged by a scorotron charging device and supplied with a constant positive surface potential E. The surface potential E is reduced almost to zero potential in the exposure portion PH in the first image exposure. A positive bias having a d.c. component extremely close to the surface potential E of the non-exposure portion is applied to a developing device to allow the reversal development, so that a first toner image (for instance, a yellow toner image) may be formed because the positively charged toner T within the developing device adheres to the exposure portion PH which has relatively low potential. Although the potential of the region where the toner image has been formed increases by DUP because of the positively charged toner T stuck thereto, the potential is further increased by CUP to provide the region with potential close to the surface potential E of the non-exposure portion as the second charging is provided by the scorotron charging device. Subsequently, the surface of the photosensitive member S wherein the roughly uniform surface potential E has been obtained is given the second image exposure to form an electrostatically charged image and a second toner image (for instance, a magenta toner image) is obtained through the same developing process as the aforementioned. By repeating the above-described process, a third toner image (for instance, a cyan toner image) is piled on the preceding ones and a multicolor toner image is obtained. The multicolor toner image is transferred to recording paper and fixed under pressure or with heat thereon and the paper is then discharged. After the transfer, the photosensitive member S is cleaned by a cleaning device in preparation for the formation of the next image.
The advantages of the aforementioned image-forming principle include the possibility of making a single transfer process sufficient, preventing the apparatus from becoming larger, image-forming time from being wasted and color toner images from being out of register.
As set forth above, use of data of the reference toner image by colors as disclosed in Japanese Patent Laid-Open No. 49-74034 may make it impossible to control the image characteristics in a desired mode when a multiple toner picture image is formed by superposing various color toner images on the image retainer. In addition to the problem above, a color image of good quality is ultimately unobtainable even though each color toner picture image is controllable, because the superposed final toner picture image is not effectively controlled.
In the apparatus for forming an image according to the Japanese Patent Laid-Open No. 49-74034, an area required for &he image retainer must be increased because the reference toner image is formed in the front position on the image retainer and the toner picture image is formed in the rear position based on the data derived from the reference toner image Consequently, the further problem is that the apparatus for forming an image is large and cannot be made compact.
In the typical apparatus for forming multicolor image, the image retainer is turned the number of times equal to that of toner images to form a plurality of toner picture images. In that case, the time consumed for recording becomes several times longer than that of an apparatus for forming a monochrome image, and this makes the formation of a multicolor image extremely inefficient.